gui, staking: Implement facilities for mandatory sidestakes and sidestake GUI #2812

Job	Run time
macOS 11 native [GOAL install] [GUI] [no depends]	10m 28s
Lint	1m 0s
ARM [GOAL install] [buster]	3m 36s
Win32	5m 39s
Win64	6m 52s
i386 Linux [GOAL install] [focal]	3m 23s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	29m 14s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	23m 9s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	16m 13s
	1h 39m 34s

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